This invention relates to screen printing, and especially to the control of screen printing apparatus which is designed for multiple-impression screen printing on a relatively high production basis.
For volume production of multiple printed copies, it is necessary that the object being printed be moved quickly from station to station in a multiple station printing apparatus, and necessary, if not critical, that the registration of the object to be printed with a particular printing screen in a progression of printing screens be maintained with reasonable accuracy. For this purpose, it has heretofore been proposed that the objects to be printed, or multiple such objects, be placed upon platens which are carried by a turntable in a circular path beneath a circular array of printing heads each located one indexing step distant from the next, with one or more stations left vacant for the unloading of the printed object from the platen and its replacement with yet another object to be printed.
It has, however, been found that certain screen printing operations do not always lend themselves to a neatly ordered time sequence of printing steps. Due largely to the difference in drying times of the varieties and amounts of inks used for screen printing, certain printing locations are not equipped with a printer, but are used to provide additional ink drying time or are equipped with heaters to cure the ink.
Currently, purchasers of multi-color screen printing apparatus purchase equipment with a larger number of printing heads than the actual number of different colors they intend to print. This is because the different artwork and the different substrates being printed require the ability to print at each sequential location and to be able to use a flash cure unit at each of these sequential locations. Additionally, the heat imparted to the ink and substrate at a flashing may interfere with the next immediate printing, because only a very short period of time has elapsed, e.g., 6-10 seconds for the ink and substrate to cool. To allow for more cooling, the station following the flashing station is left open and the printing head thereat is disabled. In large color applications such as printing with eight color, two or more flashes of ink with a subsequent cooling may be used. In order to print three or four colors, printers will often buy a six color or six head machine in order to have the flexibility to print, flash and cool at the six different stations.
After the printing apparatus is configured with the desired number of print heads, curing stations and drying stations, an operator at the loading/unloading station removes printed articles from the platens and replaces them with new articles to be printed. Should an article not be placed on a platen, the empty platen will sequence through the printing steps and be printed instead of the article. Similarly, should an article be improperly placed on the loading/unloading platen, the article will be improperly printed and its value will be destroyed. To avoid the obvious cleanup required if the platen itself is printed or to avoid printing errors caused by printing a misplaced article, a stop button can be provided which stops the indexing of the apparatus before printing occurs. Restarting the machine after a stop is difficult, since the timing relationships of the printing, drying and cooling Steps cannot always be re-achieved for partially printed articles. Accordingly, stopping and restarting the apparatus can result in lost articles.
A need exists for a multiple head printer apparatus in which the individual print heads of the apparatus can be selectively controlled on a transitory basis to inhibit printing on a given platen when doing such would produce undesired results.